1. Field of the Invention
The present invention relates to systems and methods for improving the storage efficiency of holographically written data, and in particular, to systems and methods for improving the storage efficiency of holographically written data in the context of multiple holographic write sessions.
2. Background Art
Holographic data storage (“HDS”) is a form of optical data storage, whereby recording of data is achieved by illuminating a photosensitive medium with intersecting reference and data light beams. The spatial modulation of light intensity produced by interference of the beams is recorded in a holographic data storage medium by modification of the dielectric properties of the medium, either in the form of periodic spatial modulation of the refractive index of the medium or of the absorption of the medium, to constitute a grating or a hologram. One form of holographic data storage sequentially records localized holograms each containing a single bit, and is sometimes termed micro-holographic bit storage. Another form of holographic data storage comprises volumetric page holographic recording which allows a large amount of data to be recorded in parallel in the form of a 2 dimensional bit array or data page. This is accomplished by placing a spatial light modulator in the optical path of the data light beam. The spatial light modulator imparts a data page on the data light beam by modulating the spatial profile of the object beam. Detection upon readout is typically performed by imaging the reconstructed optical data signal upon a suitable photodetector, which may comprise a single photodetector for detecting a single bit signal or a two-dimensional photodetector array for detecting a two-dimensional data page.
Examples of optically sensitive media suitable for holographic data storage include photopolymer based materials. Recent advances have produced photopolymer material with excellent optical and mechanical properties, resulting in a suitable combination of high capacity, high data read and write transfer rates, and long term data stability. However, photopolymer media typically require additional media processing steps necessary to achieve high performance stable data recording.
Photopolymer based holographic media are “saturable”, meaning that at a certain point their sensitivity decreases with the amount of exposure to light resulting in a decreased data write transfer rate during recording. The decrease in sensitivity occurs because most of the photosensitive species necessary for recording are used up. For practical applications, writing to a given region of the storage medium is stopped when a predetermined portion of photosensitive species has been consumed in order to maintain as high a data transfer rate as feasible. It is apparent that reaching this predetermined value does not mean that the medium is fully bleached or that the recording species are totally used up. It merely means that the photosensitive recording species are depleted to a sufficient extent that the internal data write transfer rate becomes lower than a predetermined minimum value. A typical threshold value may be proximate to 80% consumption of photosensitive species which equates to a proximate to 50% drop in write transfer rate from its maximum value.
Subsequent to a data recording to a region of the medium, photopolymer based holographic media are typically subjected to a “bleaching” process to “close” the region in order to render the medium stable for data readout. Closing is typically accomplished by directing a strong laser beam over the written area to complete photo reaction of remaining photoactive species. Additionally, the photopolymer based holographic media may require a photosensitization process prior to data recording for enabling high data transfer rate during actual data recording step. The process of sensitizing, recording to a region of the media and “fixing” that region is termed “a session”.
It is also well known in the prior art that multiple data holograms can be recorded within a same recording volume by means of one of a plurality of multiplexing techniques. The techniques generally involve changing one of the properties of the reference beam used to record each data hologram such that the data can be selectively retrieved only by illumination of its data storage location in the holographic data storage medium by its associated reference beam. The collection of multiple data page holograms recorded at the same data storage location is sometimes referred to as a book, and is designated as such henceforth in the present invention. Accordingly, the holographic data storage medium records a layer of multiple spatially separate books, with the spacing between data storage locations typically limited by the maximum recording beam size within the thickness of medium and recording beam alignment tolerances.
FIGS. 1A and 1B provide schematics illustrating single layered data recording on a holographic storage disk. FIG. 1A shows holographic storage disk 10 which includes single layer holographic storage medium 12. Digital data is encoded onto storage locations 14 along data tracks 16 in the manner set forth above. Disk 10 is divided into sectors such as, for example, sectors 18. Within each of sectors 18, multiple points may be proximately aligned on separate tracks 16. Blow up 20 shows that each location 14 contains multiple holograms. Typically, each hologram represents a single data page. Represented in blow up 20, each hologram has a slightly different orientation (i.e., angle multiplexing technique). FIG. 1B shows a cross-section of a region of the holographic data storage medium 12 to illustrate that multiple spatially separated books (and each containing multiple data pages) are recorded in the holographic data storage medium. The spacing between data storage locations typically is determined by the maximum recording beam size. In this case, books are not overlapping (i.e., spatially separated) and are not exactly contiguous because of system tolerances in terms of alignment, and also because of the off axis recording geometry where the beams are focused inside of the medium. Focusing allows data light beam 22 to achieve a minimum spot size 32 onto plane 34 in holographic storage medium 12. Outside of plane 34, divergence of the data light beam as well as the varying angle of incidence of the reference beam associated with each of the data pages stored into each of storage locations 14 causes an expanded recording area with a cone like shape. However, the area outside the cylinder formed around spot size 32 is only partially exposed.
The region separating neighboring data storage locations is, therefore, only partially recorded to during data recording, thus resulting in less than optimal utilization of the available recording volume of the holographic medium. To assure a region is properly closed, the curing beam must expose an area somewhat larger than the recorded area, to take into account the same beam alignment tolerances. If recording data to only a portion of a holographic storage medium, closing a session by illuminating the recorded partial region involves creating a boundary region around it. This boundary region represents lost capacity.
To further increase storage density, several techniques have been proposed for optimal utilization of the entire recording volume, see for example U.S. Pat. Nos. 5,949,558, 5,703,705 and U.S. Patent Publication No. US20040179251, which all involve the recording of multiple shifted partially overlapping books. However, their practical implementation requires certain tradeoffs. Ensuring uniform recording requires a layered recording sequence, whereby you must record a “layer” of non-overlapping holograms, before writing the next shifted layer. In this case, the capacity of a recorded region is fully utilized only when the last layer has been recorded.
With reference to FIGS. 2A, 2B, and 2C, illustrations of methods for improving the inefficiencies inherent in single layer data recording are provided. The methods of FIGS. 2A, 2B, and 2C are still further improved by combining the methods of the present invention set forth above. Holographic data disk 40 comprises a plurality of overlapping data books, such as 42, 44 and 46. As set forth above, it should be appreciated that books 42, 44, 46 are all not written in succession, but each as part of a separate recording step of a sequence of at least two recording steps, wherein during each recording step a plurality of spatially separated books are recorded in a region of the holographic medium to form a so-called layer. The recorded layer corresponding to a recording step is spatially shifted compared to the recorded layer of the at least one other recording step so that the complete recording sequence results in book overlap. It will be appreciated by one skilled in the art that these layers do not correspond to physical separate layers in the medium. In the example of FIG. 2A there is overlap between book 44 and each of books 42 and 46, but books 42 and 46 are spatially separate and recorded in succession. FIG. 2B provides a schematic of a recording sequence comprising three recording steps resulting in overlap of three data books at any given location. In this example, layers 50, 52, 54 represent layers of spatially separate books written in separate recording steps. Layer 50 includes spatially separated books 56. Similarly, layer 52 and layer 54 include, respectively, spatially separated books 58 and 60. After layer 50 is fully populated, then books 58 in layer 52 are written. Books 58 are slightly spatially shifted from books 56. When layer 52 is fully populated, then books 60 in layer 54 are written. It should be appreciated that the order in which layers 50, 52, 54 are written may be permutated as long as uniform recording conditions are maintained for the next layer to be recorded. FIG. 2C provides an illustration of resultant arrangements of overlapped books formed upon completion of the recording sequence. In practice, individual books are discriminated upon readout by inherent selectivity associated with shift multiplexing techniques or by means of filtering techniques known to those skilled in the art.
With regard to the techniques for overlapping holograms for increased volumetric efficiency and the recording sequence required for their implementation, as described above, a recording session which represents less than the total capacity of the holographic medium or that of a given portion of its recording region may result in inefficient usage of available capacity of a data recording region, by effectively recording a smaller number of recorded layers than possible. Upon closing the recorded region, the remaining recordable layers are lost in the curing process, thus negating the capacity advantage of the layered recording technique, and in the worst case reducing it to a single layer recording.
When holographic storage media is recorded in multiple sessions, the sum of the lost capacity during the closing out of each session as a result of the boundary regions between adjacent data regions can be a significant portion of the media's capacity. In the case of multi-layered holographic recording, this is further compounded by unused capacity within each region being lost during the closing out of each session. A method minimizing the capacity lost when employing multi-session recording would be desirable.
Accordingly, there exists a need for systems and methods for managing the recording characteristics of holographic data storage media which enable effective utilization of the data storage capacity of holographic storage media. In particular, there exists a need for systems and methods which enable effective utilization of the data storage capacity of holographic storage media, in a storage environment employing both single and multiple write sessions to a same storage medium.